A power supply to provide multiple outputs of different potentials is a technique known in the art. The commonly seen power supply usually has a transformer with a secondary side divided into at least one primary output circuit and a post regulation circuit connecting to the primary output circuit. The primary output circuit aims to provide primary output power which forms a greater proportion of output power through winding induction on the secondary side of the transformer. The post regulation circuit provides ancillary output power with a lower voltage and lower power by lowering the voltage of the primary output power. Take the power supply of a general desktop computer as an example, the primary output power with power and current at a greater proportion of the total output is 12V and 5V, and is delivered through two windings. The post regulation circuit is then employed to generate 3.3V.
Based on the aforesaid conventional technique, refer to FIG. 1 for a conventional post regulation circuit in which a power supply contains a transformer. The transformer has a primary side 10 with a passing current controlled by a pulse width modulation (PWM) controller outputting a PWM signal and one set of switches. The transformer has a secondary side with multiple primary output circuits 11 and 12 to generate induction power through different windings. A duty cycle signal (Duty_s) and a synchronous rectified signal (FW_O) drive a plurality of switches (Q1, Q2, Q3 and Q4) to perform synchronous rectification, and through inductors (L1 and L2) and capacitors to steady power waveforms to form two primary output power (IL1 and IL2) (12V and 5V). In addition, one set of post regulation circuit 13 comprising one set of switches 131 and 132, an inductor 133 (LPOST) and capacitors and resistors connects to the primary output circuit 12. Through the switches 131 and 132, the voltage is regulated to form an ancillary output power (ILpost) of 3.3V. The switches 131 and 132 are controlled by a post control circuit (not shown in the drawings) which outputs one set of control signals (HS and LS). By integrating current flowing through the inductor 133 to get a voltage, a driving signal can be formed through amplification of an amplifier to control the ON period of the switches 131 and 132.
Control of the present post regulation circuit relies on monitoring its induction current and performing integration of the induction current to get a corresponding voltage to regulate the ON period of the switches. Protection of the post regulation circuit is accomplished by setting an upper current limit. In the event that the induction current reaches the upper current limit, the rear edge of the driving pulse wave of the PWM controller is forcefully shrunk to prevent over current. However, when a short circuit occurs at the ancillary output power, current increases rapidly even if the rear edge of the driving pulse wave has been shrunk, the induction current of the post regulation circuit could rise instantly (about within 2-3 clocks) to generate over current 91 as shown in FIG. 2. Due to the amount of the over current 91 is excessively large, a reverse current 92 is generated on the primary output circuit connected to the post regulation circuit, and rectification elements could be burned out. This not only disables the post regulation circuit, the primary output circuit connected to the post regulation circuit also is damaged.